OP, why did you have the colonoscopy? Did they tell you what they suspected or were hoping to rule out?
I am not diagnosing you. But it sounds very similar to how my bowel presented when I first was diagnosed with Ulcerative Colitis. They explained to me what they were doing and why during the diagnosis, which seems to be sadly lacking for you.
I remember the utter terror I felt while waiting for the first appointment and thinking it was something like cancer, so I really feel for you.
I was told, when they started investigations into my symptoms, that they suspected UC, from looking just inside my bottom with a very very short endoscope. Have you had that, or did they just go straight to colonoscopy? I was told that with UC, it always starts at the end of your large intestine, and as it worsens it spreads further back. As it improves, the length of colon affected gets shorter, but always the last bit to heal up is the bit closest to your rectum. It only affects your colon. If you imagine your intestine as a tube made up of loads of layers, one inside the other, UC ulcers are in the layer that's closest to the inside, the surface they can see on a colonoscopy. It sounds like your bowel has redness and ulcers on approx the last half of your colon.
Crohns disease is similar, but can affect anywhere in your digestive tract, from mouth to bottom. It can also affect any of the layers. So the lesions might be there but completely hidden from view because they're on the middle of the sandwich, so to speak.
So when I had my first colonoscopy, they had a look, could see ulcers that looked like UC, and took biopsies. But they also took biopsies from healthy looking bits to check it wasn't Crohns, with hidden lesions. Or that there weren't lesion too small to see with the camera that would show up under the microscope.